Insulated wires are widely used for magnet coils and the like. As methods for forming an insulating layer of an insulated wire, an immersion method and an electrodeposition method are known. The immersion method is a method in which a conductive wire element serving as a core material of an insulated wire is immersed in a coating material such as a resin varnish, pulled up, and dried to form an insulating coating on the surface of the wire material. The electrodeposition method is a method in which a wire material is placed in an electrodeposition liquid including a coating material component such as a resin varnish, the wire material is used as an anode or a cathode to apply an electric current between the wire material and a counter electrode, and the coating material component is electrodeposited on the wire material surface and then subjected to a baking treatment to form an insulating layer (refer to Japanese Unexamined Publication No. S62-037396 and Japanese Unexamined Publication No. H03-241609).
The immersion method has disadvantages in that it is difficult for the coating material to attach to the corners of a flat electric wire and the layer thickness at the corners is thinner than the layer thickness at the flat portion. On the other hand, in the electrodeposition method, since the coating material is sufficiently electrodeposited even at the corners of the flat wire, there is an advantage in that it is possible to form an insulating layer in which even the corners have the same thickness as the flat portion or a greater thickness than the flat portion.
In recent years, there has been a demand for insulated wires excellent in withstand voltage strength and heat resistance in order to handle a wide range of applications and, as a means for increasing the heat resistance of an insulating layer, a coating material for enameled wires which contains metal oxide fine particles and silica fine particles in the resin of an insulating layer is known (Japanese Unexamined Publication No. 2001-307557).
However, the coating material described in Japanese Unexamined Publication No. 2001-307557 is for the immersion method and is not able to be used for the electrodeposition method when the disclosed coating material has a liquid composition or is in a liquid state. In the immersion method, it is necessary to repeat immersion and drying several times in order to obtain a desired layer thickness, for example, repeating immersion seven times in order to form a coating having a layer thickness of 35 μm which is useful in practice. Therefore, productivity is low. Furthermore, the immersion method is not able to solve the disadvantage that the layer thickness at the corners becomes thinner than that of the flat portion in a flat wire material. In addition, the environmental burden is large since an organic solvent is used as a solvent and dispersion medium of the resin and oxide fine particles.
Furthermore, in the immersion method, in a case where silica fine particles are contained in the insulating layer, since the immersion in a coating material including silica fine particles is repeated, silica fine particles are included in each layer such that the silica fine particles are in a dispersed state throughout the entire insulating layer. However, since it is the surface of the insulating layer which receives the most exposure to heat at high temperatures, the insulating layer surface is easily damaged if the concentration of silica fine particles in the vicinity of the surface of the insulating layer is small.